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Phototactic Decision-Making by Microalgae

Shantanu Raikwar1, Adham Al-Kassem1, Nir S. Gov2,3, Adriana I. Pesci4, Raphaël Jeanneret1,*, and Raymond E. Goldstein4,†

  • *Contact author: raphael.jeanneret@phys.ens.fr
  • †Contact author: R.E.Goldstein@damtp.cam.ac.uk
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Phys. Rev. Lett. 135, 228401 – Published 26 November, 2025

DOI: https://doi.org/10.1103/3fry-7tsw

Abstract

We study how simple eukaryotic organisms make decisions in response to competing stimuli in the context of phototaxis by the unicellular alga Chlamydomonas reinhardtii. While negatively phototactic cells swim directly away from a collimated light beam, when presented with two beams of adjustable intersection angle and intensities, we find that cells swim in a direction given by an intensity-weighted average of the two light propagation vectors. This geometrical law is a fixed point of an adaptive model of phototaxis and minimizes the average light intensity falling on the anterior pole of the cell. At large angular separations, subpopulations of cells swim away from one source or the other, or along the direction of the geometrical law, with some cells stochastically switching between the three directions. This behavior is shown to arise from a population-level distribution of photoreceptor locations that breaks front-back symmetry of photoreception.

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Phys. Rev. Lett. 135, 228401– Published 26 November, 2025
  • Received 2 July 2025
  • Accepted 24 October 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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